Paper No. 8
Presentation Time: 8:00 AM-6:00 PM
CHARACTERIZING THE METASOMATIC AGENT IN PERIDOTITE XENOLITHS FROM KNIPPA, TEXAS, USING TRACE ELEMENT AND ISOTOPIC COMPOSITIONS
RAYE, Urmidola1, STERN, Robert
2, LEE, Cin-Ty A.
3, KIMURA, Jun Ichi
4, HIRAHARA, Yuka
4, HANYU, Takeshi
4, MIYAZAKI, Takashi
4, QING, Chang
4, ANTHONY, Elizabeth Y.
5 and GRIFFIN, William L.
6, (1)Department of Geosciences, Universiy of Texas at Dallas, FO 21, 800 West Campbell Road, Richardson, TX 75080, (2)Geosciences Department, University of Texas at Dallas, 800 W. Campbell Rd, Richardson, TX 75080, (3)Dept of Earth Science, Rice Univ, MS 126, 6100 Main St, Houston, TX 77005, (4)Ifree-Jamstec, IFREE-JAMSTEC, 2-15 Natsushima-cho, Yokosuka, 237-0061, Japan, (5)Department of Geological Sciences, University of Texas at El Paso, El Paso, TX 79968, (6)GEMOC, Department of Earth & Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia, uraye@utdallas.edu
Spinel peridotite xenoliths in Late Cretaceous basanites from Knippa, TX are the only samples of Texas continental lithospheric mantle. The xenoliths are typically medium-grained spinel lherzolites and harzburgites with equigranular textures and triple junction grain boundaries. Most are fresh but some have serpentine veins. Mineral assemblages and compositions (olivine Fo
89-92, orthopyroxene En
88-91, Cr-diopside En
45-49Fs
3-5Wo
40-45, spinel Cr# = 15 - 36) record at least one depletion event caused by melt extraction. Primitive mantle normalized whole-rock data for fresh as well as serpentinized samples are variably enriched in light rare earth elements (LREE; (La/Yb)n = 0.26 – 17.1) with flat heavy rare earth element (HREE) patterns (0.80-0.93 xPM). Whole-rock REE as well as in clinopyroxenes show two patterns: (1) LREE enriched (La/Yb)n = 3.47-25.7; (2) spoon shaped (La/Yb)n = 1.97-27.6 and overall flat HREE distributions (1.03-3.63 xPM).
Modeling whole-rock and clinopyroxene trace element abundances shows that the lherzolites can be produced by 3-9% fractional melting whereas harzburgites require 10-15% fractional melting of primitive mantle and agree with estimates based on spinel compositions. Batch melting is not realistic because this requires harzburgites to be produced by 27-30% melt extraction. This should exhaust CPX but Knippa harzburgites contain < 5% CPX. Clinopyroxene Nd and Hf isotopic data show that the Sm-Nd system was disturbed, whereas Lu-Hf results give an age of 359±90 Ma, indicating that the Lu-Hf isotopic composition was completely reset by the Pennsylvanian Ouachita orogeny (~ 350 Ma) during the final stages of assembly of Pangea. LA-ICP-MS Re-Os analyses (TRD 0.28-1.1 Ga and TMA 0.42- 1.42 Ga) on interstitial sulphides also indicate that Mesoproterozoic mantle was reset by a Late Paleozoic event, which may have been metasomatic. PM-normalized trace element troughs at Nb-Ta, Zr-Hf and Ti suggest carbonate melt metasomatism. However, (La/Yb)n vs Zr/Zr* as well as (La/Yb)n vs Ti/Eu indicate carbonate and silicate metasomatism.
